To increase the production of an oil, gas, geothermal, or other type of well, the producing zone of the geological formation surrounding the well is fractured to allow the desired fluids to flow more freely through the formation and into the well. Fluid is pumped into the formation under high pressure to fracture the producing zones. However, if fracturing fluid is pumped into the formation during the fracturing operation without some accompanying solid, the geological formation pressures will cause the fractured areas of the formation to close when the pumping of fracturing fluid stops, thus restricting the flow of the oil or gas.
A slurry of particulate material, such as sand blended with the fracturing fluid, may be forced into the fissures in the geological formation to keep the formation open after the slurry has been pumped into the well. Well servicing equipment incorporates blending apparatus to mix the particulate material with the fracturing fluid. The blender discharges the slurry to a high pressure, downhole pump that injects it into the well and into the producing zones. It is important that the discharge pressure of the blender remains constant to prevent the downhole pump from cavitating, a condition in which inlet fluid flow is reduced or air is passed through the pump and downhole pressure is lost. When cavitation occurs in the downhole pump, the fracturing operation fails.
It is desirable to use multiple blending tubs in the blending and fracturing operations. Multiple blending tubs increase the flow rate and provide a failsafe backup system in the event that one of the tubs fails. However, because of cavitating and other downhole pump problems, it has been difficult to use multiple tubs simultaneously. It is crucial to a cost effective fracturing operation that a high flow rate of slurry is reliably delivered at a relatively constant pressure to the downhole pumping equipment.